add History data structure

GitOrigin-RevId: b327e9a0a43abff8aa53f4531e0168f99c38c46b

Jumpy.Shared/History.cs

@@ -0,0 +1,53 @@
+using System;
+
+namespace Jumpy {
+  // A History is a queue of fixed length N that records the N most recent items Add()ed to it.
+  // The mostly-recently-added item is found at index 0; the least-recently-added item is at index
+  // N-1. Items older than the History's size are automatically dropped. The underlying
+  // implementation is a fixed-size circular array; insertion and access are O(1).
+  //
+  // Example:
+  //   h = new History<int>(3);
+  //   h.Add(2); h.Add(3); h.Add(5);
+  //   Console.WriteLine("{0} {1} {2}", h[0], h[1], h[2]);  // 5 3 2
+  //   h.Add(7);
+  //   Console.WriteLine("{0} {1} {2}", h[0], h[1], h[2]);  // 7 5 3
+  //   h.Add(11); h.Add(13);
+  //   Console.WriteLine("{0} {1} {2}", h[0], h[1], h[2]);  // 13 11 7
+  class History<T> {
+
+    // Backing store for the History's items.
+    private T[] items;
+    // Points at the most-recently-inserted item.
+    private int idx = 0;
+
+    public History(int length) {
+      items = new T[length];
+    }
+
+    public void Add(T item) {
+      idx++;
+      if (idx >= items.Length) {
+        idx -= items.Length;
+      }
+      items[idx] = item;
+    }
+
+    public int Length {
+      get => items.Length;
+    }
+
+    public T this[int age] {
+      get {
+        if (age < 0 || age >= items.Length) {
+          throw new IndexOutOfRangeException();
+        }
+        int lookup = idx - age;
+        if (lookup < 0) {
+          lookup += items.Length;
+        }
+        return items[lookup];
+      }
+    }
+  }
+}

Jumpy.Shared/Jumpy.Shared.projitems

@@ -11,6 +11,7 @@
   <ItemGroup>
     <Compile Include="$(MSBuildThisFileDirectory)Camera.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)FpsCounter.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)History.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)IDisplay.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)KeyboardInput.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)JumpyGame.cs" />